Allow to archive labels (#26478)

## Archived labels 

This adds the structure to allow for archived labels.
Archived labels are, just like closed milestones or projects, a medium to hide information without deleting it.
It is especially useful if there are outdated labels that should no longer be used without deleting the label entirely.

## Changes

1. UI and API have been equipped with the support to mark a label as archived
2. The time when a label has been archived will be stored in the DB

## Outsourced for the future

There's no special handling for archived labels at the moment.
This will be done in the future.

## Screenshots

![image](208f95cd-42e4-4ed7-9a1f-cd2050a645d4)

![image](746428e0-40bb-45b3-b992-85602feb371d)

Part of https://github.com/go-gitea/gitea/issues/25237

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/org/label.go

@@ -209,6 +209,7 @@ func EditLabel(ctx *context.APIContext) {
 	if form.Description != nil {
 		l.Description = *form.Description
 	}
+	l.SetArchived(form.IsArchived != nil && *form.IsArchived)
 	if err := issues_model.UpdateLabel(l); err != nil {
 		ctx.Error(http.StatusInternalServerError, "UpdateLabel", err)
 		return